drm/i915: Set correct TV detection voltage level override values

We detect TV connect status by setting DAC voltage level override
values as 0.7 voltage for DAC_A/B/C. The corresponding 2-bits shold be 0x2,
In order correctly to set last bit as 0, at first we must clean it.

It fixed freedesktop.org bug #21204

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>